Liquid crystal displays have expanded their market as small-size displays for watches, cellular phones, etc. and as notebook-type personal computer monitors and desktop-type personal computer monitors taking the place of CRTs (Cathode Ray Tubes) due to their features of light weight, low profile and low power consumption. Further, recently, applications of liquid crystal displays have been expanded as monitors or TV sets supporting DVD playback for displaying not only still images but also moving images. Thus, the applications range widely.
However, when moving images are displayed in a liquid crystal display serving as a personal computer monitor in the background art, there appears an image quality deterioration such as a phenomenon (hereinafter referred to as “moving image blurring”) that the display contents are blurred. Thus, the performance of the liquid crystal display is inferior to that of a CRT.
This moving image blurring has been described in detail, for example, in “Image Quality in Displaying Moving Images in Hold-type Display” (IEICE Technical Report, EID 99-10, p. 55), which suggests that the moving image blurring is largely caused by a difference in display system between a CRT and a liquid crystal display.
That is, the moving image blurring is caused by a hold-type display system in which constant luminance is always kept within one frame in the liquid crystal display, as compared with an impulse display system in which lighting is performed at only a certain moment in one frame in the CRT. It is suggested that the moving image blurring appears in the hold-type display system due to a difference between an integral path assumed by the display system and an integral path based on a human sight line.
Accordingly, when a display system close to an impulse-type display system as in the CRT is used in the liquid crystal display, it is possible to improve the moving image blurring caused largely by the hold-type display system. As a specific means for such a solution, there has been proposed a blink backlight system in which a backlight serving as a light source of a liquid crystal display is lit like pulses in one display period so as to realize a pseudo impulse display system.
Some liquid crystal displays using this blink backlight system have been already manufactured by way of trial, and it has been confirmed that the moving image blurring can be improved. For example, the details have been described in “Image Quality Revolution” ‘Flat-Panel Display 2002, p. 96) and so on.
Here, FIG. 19 shows a schematic diagram of a background-art liquid crystal display using a blink backlight system. In FIG. 19, the liquid crystal display is constituted by a power supply 1; a video signal processing circuit 2; a control circuit 3; a drain driver 4; a gate driver 5; a common driver 6; a liquid crystal panel 7; a light source drive circuit 11 having an inverter 8, a blink signal generation circuit 9 and a switching device 10; and a light source 13 formed out of light emitting tubes 12.
Each light emitting tube 12 forming the light source 13 of the liquid crystal display is typically made of a three band fluorescent tube 12. The fluorescent tube is connected to the inverter circuit 8. The on/off timing of a voltage or a current supplied from the inverter circuit to the fluorescent tube is controlled by a signal (hereinafter referred to as “blink signal”) from the blink signal generation circuit 9.
For example, as shown in FIG. 20, a current value I is applied into the fluorescent tube in accordance with a signal from the inverter circuit 8 at the timing T=T0 when the blink signal is turned on. Thus, the fluorescent tube is also turned on at the same time. At the timing T=T1 when the blink signal is turned off, the current becomes zero. Thus, the fluorescent tube is also turned off at the same time.
In such a manner, the backlight is blinked within one frame so that a turned-on state and a turned-off state are set. Thus, a pseudo impulse display system close to the CRT display system is attained so that blurring of moving images can be improved. Accordingly, the blink backlight system is a very important technique in a moving image supporting liquid crystal display such as a liquid crystal television set.
However, when the backlight is blinked like pulses within one frame, there occurs a new problem. That is a problem of “color misregistration” appearing in a moving image contour portion.
For example, assume that a black window 15 is displayed on a white background 14 and this black window 15 is moved from the left side of the screen to the right side, as shown in FIG. 21. In this event, of the black window contour portion, a contour portion 16 perpendicular to the black window moving direction 17 has color fringing (color misregistration).
This “color misregistration” phenomenon will be described below.
A three band fluorescent tube is generally used as a light source of a liquid crystal display as described previously. The three band fluorescent tube is filled with discharge gas such as mercury gas, and three color (R, G and B) fluorescent materials are applied to the inner wall of the tube. As soon as a current is applied to the fluorescent tube, discharge breaks out inside the tube, and ultraviolet rays 254 nm long are chiefly emitted due to the mercury gas. The ultraviolet rays excite the fluorescent materials applied to the inner wall of the fluorescent tube so that three color (R, G and B) lights are emitted from the fluorescent materials to the outside of the tube.
Accordingly, the fluorescent materials are ideally blinked in accordance with the on/off switching of the current. In fact, however, due to the response speed of each fluorescent material itself, it takes time for the fluorescent material to reach its intended luminance after the fluorescent material is irradiated with the ultraviolet rays, as shown in FIG. 22. Further, the response time differs from one fluorescent material (R, G, B) to another. Thus, color is shifted in the rise time of luminance. In addition, as for the fall time of luminance, the time for the luminance to fall (turn off) differs from one fluorescent material to another in the same manner. Thus, color is shifted likewise.
Incidentally, the rise time of luminance here means a time (T0<T<T3) required for enough rise of the luminance of a fluorescent material having the slowest response speed after the current is turned on. On the other hand, the fall time of luminance means a time (T1<T<T4) required for enough fall of the luminance of the fluorescent material having the slowest response speed after the current is turned off.
At present, Y2O3:Eu, LaPO4:Tb,Ce and (Ba,Sr)MgAl10O17:Eu are used as fluorescent materials of the three colors, that is, red, green and blue respectively in a typical three band fluorescent tube. Of these fluorescent materials, the blue fluorescent material typically has the fastest response speed. The luminance rise response time and the luminance fall response time of (Ba,Sr)MgAl10O17:Eu are shorter than 1 msec. The luminance rise response time and the luminance fall response time of the red Y2O3:Eu range from 3 msec to 4 msec, and those of the green LaPO4:Tb,Ce are longer, ranging from 6 msec to 7 msec. Incidentally, the luminance rise response time here means a time required for the luminance to reach 90% from 0% on the assumption that the ultimate luminance is 100%. On the other hand, the luminance fall response time here means a time required for the luminance to reach 10% from 100%
For example, when the fluorescent tube has a characteristic shown in FIG. 22, the fluorescent tube emits white light with a bluish tone in the luminance rise time, and white light with a greenish tone in the luminance fall time.
When the backlight is blinked like pulses within one frame in order to improve blurring of moving images, the blurring of moving images can be improved, but there occurs a new problem of color misregistration due to blinking of the backlight.